Anorectal Abscess and Fistula in Ano

Introduction

Considering the frequent exposure to a large bacterial load and high pressure, infection of the anorectal crypts with a resultant abscess or fistula is relatively uncommon. Limited epidemiologic data indicate an annual incidence of about 9 cases per 100,000 in the population, most commonly affecting people aged 30 to 50 years. Approximately 30% to 50% of patients with an initially treated abscess will experience a subsequent abscess recurrence or frank anorectal fistula formation. Male gender, smoking, diabetes, and inflammatory bowel disease (IBD) are risk factors for the initial anorectal abscess formation. However, recurrence is not associated with gender, smoking, human immunodeficiency virus (HIV) status, sedentary lifestyle, or perioperative antibiotic use according to the majority of studies. Instead, the single most important predictor of recurrence or fistula formation is age under 40 years. Other possible predictors of recurrence, supported by studies in widely divergent settings, include infection with Escherichia coli and the absence of diabetes mellitus.

Etiology and Clinical Features

In more than 90% of cases, the pathogenesis of anorectal abscess formation is thought to be anal crypt obstruction by inspissated mucus or stool ( Fig. 49.1 ). As trapped bacteria proliferate and mucus and pus accumulate, an abscess forms and erodes through adjacent tissue planes, resulting in classic signs of tenderness, redness, swelling, and heat. Thus most patients become aware of their symptoms later in the stages of abscess formation. Patients will generally complain of throbbing or dully aching pain that is aggravated by walking, sitting, straining, coughing, and sneezing.

Abscess progression can proceed in any direction. If the abscess is eroding superiorly or in an intersphincteric manner, a swollen mass may not be obvious. Urinary retention, fever, or even septicemia may accompany the anorectal pain and is an urgent and important clue to the presence of an abscess in cases of cephalad or otherwise obscure infectious erosion. Fortunately, in most cases, the abscess will erode toward the perianal margin. Without intervention, most abscesses eventually will rupture through the anal margin skin. Formation of this external opening provides tremendous relief of pain, as well as some anxiety to the patient. The initial cryptoglandular insult results in abscess, but it is the persistent internal opening of the initial crypt which results in fistula in ano.

History and Physical Examination

During the initial consultation, a thorough history is paramount and should include documentation of any previous similar events; previous anorectal surgery or other trauma; previous obstetric injury; previous history of sexual assault; personal or family history of IBD or colorectal cancer; symptoms consistent with potentially undiagnosed IBD such as unintentional weight loss, chronic diarrhea, or abdominal pain; and symptoms consistent with lymphoma, leukemia, or HIV infection such as weight loss, night sweats, lymphadenopathy, or unexplained fevers. Obtaining and documenting a thorough history will provide guidance for an appropriately aggressive treatment plan and potential use of medical as well as surgical therapies.

Careful documentation of bowel habits is also prudent and should include frequency of defecation; fecal urgency or incontinence to gas, liquid, or solids; presence of pain or bleeding with defecation; and sexual dysfunction. If there are any concerns regarding the risk of colorectal cancer, such as bleeding per anus, or changes in stool caliber, the patient should be referred for colonoscopic evaluation after the anorectal pain and infection have been addressed.

A comprehensive physical examination must be conducted to rule out underlying or concomitant diseases. The examination of the perineum, often referred to in the medical record as the “rectal exam,” should be preceded by ensuring patient privacy and respectful treatment, and should include an additional medical staff person in the examination room. The most important part of the physical examination is alerting the patient before physically touching the perineum.

Documentation of the physical examination should include the appearance of the perineum, specifically the condition of the skin; presence of erythema; presence of abnormal pigmentation, papular lesions, or masses; and potential perianal soiling, which can indicate compromised continence. It is important to examine the perineum anterior to the anus including the intertriginous folds between the perineum and thighs. In the presence of induration, an inflamed mass, or an external opening, the location should be documented as “posterior,” “anterior,” “right,” and/or “left.” Describing lesions using a clock face can be very confusing in subsequent examinations, during which the patient may be in prone, supine, or lateral positions.

After visualization, the anus and perineum should be tested for neuromuscular function if the patient is not excessively tender . The presence of an intact sacral spinous pathway is documented by the presence of an “anal wink” with light touch (after warning the patient). Previously noted erythema, induration, or external fistula opening should be palpated for the presence of a firm cord of tissue that can help define a fistulous tract. The gluteal muscles should be distracted to examine for the presence of an anterior or posterior midline fissure. This maneuver also helps identify whether the patient is too tender to tolerate the insertion of a finger into the anal canal or digitation. If the patient is unable to tolerate digitation, 2% viscous lidocaine can be applied or the examination should be conducted in a setting that permits sedation or general anesthesia. If the patient is able to tolerate digitation, a finger should be gently and slowly inserted into the anal canal after verbally warning the patient that his will happen. After noting baseline anal sphincter tone, the examiner should request that the patient squeeze then relax the anal sphincter. This permits the assessment of anal sphincter function; it is particularly important to note a diminished squeeze in the medical record. The patient should also be asked to bear down and then relax. This request may be confusing and it can be helpful to repeat and explain. The puborectalis muscle will feel like a thick band posterior and just cephalad to the anal sphincter. The puborectalis muscle should relax when the patient bears down. If the puborectalis muscle tightens instead, the patient may have paradoxic puborectalis function, thus substantially increasing anal canal pressures during defecation and mechanically promoting cryptoglandular infection. Finally, the distal rectum should be palpated to search for evidence of a supralevator fluctuance or tenderness that would require intraoperative drainage.